The ultimate points of origin of the nerves from the brain, are hitherto hard ly determined ; so that it is still question ed, whether or no the right and left nerves decussate. That this is the case in the optics is tolerably clear ; and the fact, that injury of one side of the brain causes paralysis of the opposite side of the body, has led to an inference, that the same decussation obtains in all the nerves. The ganglia and plexuses, in which diffe rent nerves are united together, proba bly perform an office analogous to the arterial communications ; that of pre serving the connection of any part with the brain, when the direct communica tion is cut off.
Physiologists have endeavoured to trace the termination of nerves in the organs which they supply ; but the research is almost too subtle for our imperfect modes of investigation. In some instances, as the optic and auditory, they are manifest ly resolved into a soft pulp : and we con lecture, by analogy, that the same mode may obtain in others. There are some obvious differences, which may account for the different mode of affection in the various senses. The retina, which is the expanded end of the optic nerve, is of so delicate pulpy a nature, as to approach to fluidity, and it is acted on only by the rays of light, which are not perceived by any other sense : the auditory nerve, which alone perceives sounds, is rather firmer, &c.
That the mind is very immediately and essentially connected with the brain, and that the animal functions of sensation and voluntary motion, are no less intimately dependant on the same organ, may be proved by such an abundance of physio logical and pathological phenomena, that no doubt can be entertained of the fact. An injury of this organ suspends or an nihilates the whole, or a part, of the men tal operations, and puts an end to all feel , ing and motion : the organs of the body remain entire, the nerves connecting these with the sensorium are uninjured ; but the perceptive faculty is lost. Again, an injury of one side of the brain often causes a loss of feeling and motion in one side only of the body ; which, in consequence of principles inexplicable by us, always affect the opposite half to the injury. That the sensible impressions made on our organs are conveyed by the nerves to the brain ; and that the latter part is the seat of the sensation, although it is refer red by the mind to the part itself; is proved by cutting or tying a nerve : in which case, the usual impression causes no perception. The truth of this asser tion, which will hardly meet with credit among the uninformed, is illustrated by what happens to persons whose limbs have been amputated : they are constant ly complaining of pains in the toes or fin gers of the limbs they have lost. Here
the middle of the nerve is irritated, but the pails is referred by the mind to its ex tremities.
Yet, although the influence of the brain be thus essential, in the business of sen sation and voluntary motion, and an un impaired state of the nerves passing be tween the organ and the sensorium, be consequently an indispensable condition in those functions, other departments of the animal economy are not so immediately subject to the power of the brain. The processes of digestion, absorption, circu lation, secretion, and nutrition, those, in short, which constitute the internal life, still go on, when injuries of the brain have suspended the animal functions : nay, they may survive for months or even years. The ligature of the nerves of a part does not destroy its circulation or nutrition ; although these processes may perhaps be impaired. How, then, will it be said, does an injury of the brain so often prove fatal ? The individual ought still to live internally, although his external life has been annihilated. But here we notice a function that partakes of both : namely, respiration. The dilatation of the chest can only be performed by means of mus cles, whose principle of action comes from the brain: as the injury of the latter organ has paralysed these, the blood can no lon ger receive those changes which it under goes in respiration, and thereby becomes unfit to stimulate the heart to action, or to keep up the powers of life in any of the organs of the body.
That the nerves are, as we have describ ed, the medium of connection between the mind and its organs, is clear ; but how their offices are performed, is a much more obscure question. It has occupied the attention, and engaged the experi ments of physiologists, in all ages ; but nature has not hitherto lifted the veil. and the subject remains nearly in its original obscurity. An oscillatory or vibratory motion of the nerves, or a nervous fluid contained in or adhering to these organs, have been assumed in explanation of the facts. According to some, the latter is a liquid contained in tubes ; while others liken it to caloric, light, oxygen, the elec tric, or magnetic fluids. The partisans of the latter opinions consider, that the recent discoveries of galvanism add much weight to their arguments. See GA L VA.